This invention relates to nickel base alloys, and particularly, though not exclusively, to alloys suitable for use in compressor and turbine discs of gas turbine engines. Such discs are critical components of gas turbine engines, and failure of such a component in operation is usually catastrophic.
There is a continuing need for improved alloys to enable disc rotors in gas turbine engines, such as those in the high pressure (HP) compressor and turbine, to operate at higher compressor outlet temperatures and faster shaft speeds. In addition, high climb ratings are increasingly required by commercial airlines to move aircraft more quickly to altitude to reduce fuel burn and to clear the busy air spaces around airports, which means that the time the engines must spend at maximum power is significantly increased. These operating conditions give rise to fatigue cycles with long dwell periods at elevated temperatures, in which oxidation and time dependent deformation significantly influence the resistance to low cycle fatigue. As a result, there is a need to improve the resistance of alloys to surface environmental damage and dwell fatigue crack growth, and to increase proof strength, without compromising their other mechanical and physical properties or increasing their density.
Known alloys cannot provide the balance of properties needed for such operating conditions, in particular damage tolerance performance under dwell cycles at temperatures in the range of 600° C. to 800° C., resistance to environmental damage, microstructural stability and high levels of proof strength. As such, they are not good candidates for disc applications at peak temperatures of 750° C. to 800° C., because component lives would be unacceptably low.
Some known nickel base alloys have compromised resistance to surface environmental degradation (oxidation and Type II hot corrosion) in order to achieve improved high temperature strength and resistance to creep strain accumulation, and in order to achieve stable bulk material microstructures (to prevent the precipitation of detrimental topologically close-packed phases).
Turbine discs are commonly exposed to temperatures above 650° C., and in future engine designs will be exposed to temperatures above 700° C. As disc temperatures continue to increase, oxidation and hot corrosion damage will begin to limit disc life. There is therefore a need, in the design of future disc alloys, to prioritise resistance to oxidation and hot corrosion ahead of other properties.
Without suitable alloys, environmental protection will need to be applied to discs, which is undesirable and technically very difficult.